Make box_ not Option in block flows.(fixes #2012)
Destroy LayerBuffers before dropping them

This should fix a potential pixmap leak and task failure when the LayerBuffer
is dropped (#1187).
Remove unused create_layer_group_for_pipeline
Don't reuse an old pipeline for a new root layer

When navigating to a new URL, we need to make sure compositor messages go to
the new page's pipeline, not the previous one.
Fix HTMLElementDerived::is_htmlelement to take non-HTML elements into account.
auto merge of #2068 : mbrubeck/servo/1187-buffer-destroy, r=larsbergstrom

This fixes a potential pixmap leak and task failure when the LayerBuffer is dropped (#1187).

I wasn't able to reproduce the failure using the steps from #1187, but based on the stack trace I think this is the correct fix.  Alternately, would it make sense to `impl Drop for LayerBuffer`?
auto merge of #2020 : lpy/servo/issue2012, r=larsbergstrom

see #2012
auto merge of #2084 : mbrubeck/servo/navigate-new-pipeline-fix, r=larsbergstrom

After clicking a link to load a new page, the new page content is never painted.  This is because the compositor is still sending render messages to old frame tree's pipeline, not the new one.

This patch fixes this by ensuring that the compositor's new root layer is attached to the correct pipeline. (Note: You will also need to apply #2080 if you want to test this without crashing.)
auto merge of #2086 : Ms2ger/servo/htmlelement-cast, r=jdm
auto merge of #2088 : Ms2ger/servo/global-ToJSValConvertible, r=jdm
Make GetCallableProperty more rustic.